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  <div class="titlepage"><div><div><h1 style="clear: both">C Macro API Reference</h1></div></div></div>
  

<p>The C macro API header <code>&lt;boost/outcome/experimental/result.h&gt;</code> consists of these macros:</p>

<dl>
<dt><code>BOOST_OUTCOME_C_DECLARE_RESULT(ident, T, E)</code>
<dd>Declares to C a <code>basic_result<T, E></code> type uniquely
identified by <code>ident</code>. <code>T</code> is available at the
member variable <code>.value</code>, and <code>E</code> is available
at the member variable <code>.error</code>.

<dt><code>BOOST_OUTCOME_C_RESULT(ident)</code>
<dd>A reference to a previously declared <code>result</code> type with
unique <code>ident</code>.

<dt><code>BOOST_OUTCOME_C_RESULT_HAS_VALUE(r)</code>
<dd>Evaluates to 1 (true) if the input <code>result</code> has a value.

<dt><code>BOOST_OUTCOME_C_RESULT_HAS_ERROR(r)</code>
<dd>Evaluates to 1 (true) if the input <code>result</code> has an error.

<dt><code>BOOST_OUTCOME_C_RESULT_ERROR_IS_ERRNO(r)</code>
<dd>Evaluates to 1 (true) if the input <code>result</code>'s error value
is a code in the POSIX <code>errno</code> domain.
</dl>

<p>The above let you work, somewhat awkwardly, with any C-compatible
<code>basic_result&lt;T, E&gt;</code>. <code>basic_result&lt;T, E&gt;</code> is trivially copyable and
standard layout if its <code>T</code> and <code>E</code> are both so, and it has the C layout:</p>
<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">struct</span> <span class="n">cxx_result_</span><span class="cp">##ident
</span><span class="cp"></span><span class="p">{</span>
  <span class="k">union</span>
  <span class="p">{</span>
    <span class="n">T</span> <span class="n">value</span><span class="p">;</span>
    <span class="n">E</span> <span class="n">error</span><span class="p">;</span>
  <span class="p">};</span>
  <span class="kt">unsigned</span> <span class="n">flags</span><span class="p">;</span>
<span class="p">};</span>
</code></pre></div>
<h3 id="system-error2-support"><code>&lt;system_error2&gt;</code> support</h3>

<p>Because erased status codes are not trivially copyable and
therefore do not have union based storage, we have separate C macros
for results whose <code>E</code> is an erased status code:</p>

<dl>
<dt><code>BOOST_OUTCOME_C_DECLARE_STATUS_CODE(ident, value_type)</code>
<dd>Declares to C a status code type with domain <code>value_type</code>
available at the member variable <code>.value</code>. The <code>ident</code>
must be any identifier fragment unique in this translation unit. It is
used to uniquely identify this status code type in other macros.

<dt><code>BOOST_OUTCOME_C_STATUS_CODE(ident)</code>
<dd>A reference to a previously declared status code type with unique
<code>ident</code>.

<dt><code>BOOST_OUTCOME_C_DECLARE_RESULT_STATUS_CODE(ident, T, E)</code>
<dd>Declares to C a <code>basic_result<T, E></code> type uniquely
identified by <code>ident</code>. <code>T</code> is available at the
member variable <code>.value</code>, and <code>E</code> is available
at the member variable <code>.error</code>.

<dt><code>BOOST_OUTCOME_C_RESULT_STATUS_CODE(ident)</code>
<dd>A reference to a previously declared <code>result</code> type with
unique <code>ident</code>.
</dl>

<p>There is a high likelihood that C++ functions regularly called by C
code will return their failures either in erased <code>system_code</code>
or in <code>posix_code</code> (i.e. <code>errno</code> code domain). Via querying the
returned value using <code>BOOST_OUTCOME_C_RESULT_ERROR_IS_ERRNO(r)</code>, one can determine
if the returned code is in the <code>errno</code> code domain, and thus can be
fed to <code>strerror()</code> and so on. Therefore there are
convenience macro APIs for those particular use cases.</p>

<dl>
<dt><code>BOOST_OUTCOME_C_DECLARE_RESULT_ERRNO(ident, T)</code>
<dd>Declares to C a <code>basic_result&lt;T, posix_code&gt;</code>
type uniquely identified by <code>ident</code>.

<dt><code>BOOST_OUTCOME_C_RESULT_ERRNO(ident)</code>
<dd>A reference to a previously declared <code>basic_result&lt;T, posix_code&gt;</code>
type with unique <code>ident</code>.

<dt><code>BOOST_OUTCOME_C_DECLARE_RESULT_SYSTEM(ident, T)</code>
<dd>Declares to C a <code>basic_result&lt;T, system_code&gt;</code>
type uniquely identified by <code>ident</code>.

<dt><code>BOOST_OUTCOME_C_RESULT_SYSTEM(ident)</code>
<dd>A reference to a previously declared <code>basic_result&lt;T, system_code&gt;</code>
type with unique <code>ident</code>.
</dl>


        </div><p><small>Last revised: December 17, 2020 at 11:27:06 UTC</small></p>
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